This invention relates to a process for controlling the molecular weight of alternating copolymers which polymerize through radical initiation of charge transfer complexes of the electron rich and electron deficient comonomers using a combination of solvents to either raise or lower the molecular weight to a desired level.
The role of charge-transfer complexes in alternating copolymerization is not entirely clear but this theory does account for the rapid copolymerization of monomer pairs which individually do not polymerize under radical initiation. Bartlett and Nozaki [JACS, 68, 1495 (1946)] introduced the theory of polymerization of complexes. Seiner and Litt [Macromol. 4, No. 3, 308 (1971)] supported this theory and proposed that changing solvents and monomer concentrations would charge the equilibrium constant for complex formation, the copolymer reactivity ratios, and the dielectric constant of the entire solution.
While there are several known techniques for controlling the molecular weight of radical polymers, it has been observed that the alternating copolymerizations do not behave as typical radical addition polymerizations. The common control techniques have some effect but the magnitude is often not of the expected order. Typical techniques for molecular weight modification well known to polymer chemists include: changing the polymerization temperature, initiator concentration, and adding chain transfer agents. The effective use of chain transfer agents to lower the molecular weight of an alternating copolymer is exemplified by Johnson, U.S. Pat. No. 2,913,437. This method, however, yields a slurry of precipitated polymer and would not be very useful if the application for the polymer called for a solution of dissolved copolymer. In going the other direction, that of obtaining high molecular weight copolymers, Hibbard, U.S. Pat. No. 2,872,436 used methylene chloride as the polymerization solvent and Verdol and Thienot, U.S. Pat. No. 3,423,355, used low temperatures and low levels of very reactive alkyl peroxy dicarbonate initiators and a ketone solvent.
The above methods for altering the molecular weight of alternating copolymers are effective but the necessary conditions are quite restrictive. The process of Hibbard is not desirable since it yields only high molecular weight copolymers and the process of Johnson presents a fire hazard in handling the large amounts of aromatic chain transfer agents.